Method of making a porous ceramic filter

ABSTRACT

A process for manufacturing a porous Si 3  N 4  ceramic filter, wherein silicon powder, the raw material, methyl cellulose, the binding agent, and water, the solvent, are well-mixed together to obtain a slurry. A sponge is then dipped into the slurry to adsorb the slurry. After drying the sponge in an oven, it is biscuit fired at 350° C. for 2 hours, and then sintered at 1200-1400° C. for 15 hours.

BACKGROUND OF THE INVENTION

The present invention relates to a process for manufacturing a porousceramic filter using silicon powder as a raw material and methylcellulose as a binding agent.

In general, a porous ceramic filter can be used in metallic smeltingprocesses to filter off impurities in molten metals. In the past, thematerial used for making ceramic filters was aluminium oxide, but it isonly suitable for filtering molten aluminium alloys due to its lowresistance to heat. If Si₃ N₄ (silicon nitride) is used as a rawmaterial, the thermal shock resistance of the ceramic filter produced isthree times that of filters using aluminium oxide, therefore, a Si₃ N₄ceramic filter can be utilized in stainless steel or superalloyprocesses with operating temperatures higher than 1400° C.

PCT No: WO 88/00933 discloses a method for fabricating a porous ceramicfilter with silicon powder combined with silicon dioxide as a startingmaterial. After injection molding or extrusion molding, the organicadditives in the material are burnt off at 800° C., then the ceramiccomposition is nitrided to silicon nitride. However, the process iscomplicated since several kinds of organic compounds are added and hightemperatures (e.g.800° C.) are required to totally burn off theadditives.

German Patent Publication No. DE 38 35 807 uses silicon nitride asstarting material, and adds aluminium oxide, yttrium oxide, siliconoxide, titanium dioxide, magnesium oxide, zirconium oxide and chromiumoxide as sintering agents. After a ceramic slurry is readily prepared,it is adsorbed to a foamed polymer to form green ware. Finally, thegreen ware is dried and sintered at 1500°-1800° C.

In German Patent Publication No. DE 39 05 080 a slurry is adsorbed to afoamed organic material, such as polyurethane foam, with adsorptionthickness of 0.5-3 mm, then it is subjected to a sintering process.

In Japanese Patent Publication No. JP 01 280021 a ceramic compositionincluding aluminium oxide, yttrium oxide, magnesium oxide, silicon oxideand silicon nitride is biscuit fired at 500°-900° C., and sintered at1400°-1900° C.

Japan Laid Open No:82-10-7215 discloses a composite ceramic includingaluminium oxide, cordierite, silicon carbide and silicon nitride, andthe ceramic product is used for filtering the carbon powder in the air.

Besides silicon nitride, zirconium oxide can also be used as a materialfor producing porous ceramic filter that are resistant to temperaturesgreater than 1400° C. However, the price of zirconium oxide is muchhigher than that of silicon powder.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processfor sintering a porous ceramic filter that can be used in processesoperating at temperatures higher than 1400° C.

It is another object of the present invention to provide a process formanufacturing a porous Si₃ N₄ ceramic filter using silicon powder, whichis three times less expensive than silicon nitride, as the startingmaterial.

According to the present invention, a ceramic slurry is prepared usingsilicon powder as the raw material, water as the solvent, and methylcellulose as the binding agent. Subsequently, the slurry is adsorbedonto a sponge made of thermal plastic material. The sponge is biscuitfired by heating to 350° C. to remove the organic components, such asthe C--C and C--H bonds, in the ceramic composition. The porous body isthen sintered at 1200°-1400° C. to produce a porous ceramic filter.

DETAILED DESCRIPTION OF THE INVENTION

The objectives, features, and advantages of the present invention can bebetter understood from the following examples and accompanying figures,wherein:

FIG. 1 is a graph of the amount of methyl cellulose added vs. slurryviscosity of the porous Si₃ N₄ ceramic filter in Example 1; and

FIG. 2 is an XRD spectrogram of the porous Si₃ N₄ ceramic filter inExample 1.

Example 1

Silicone powder is well-mixed with a suitable amount of water (66.7 wt %dry solid), and 0.3 wt % (relative to the weight of water) of methylcellulose to obtain a slurry with viscosity of about 20,000 c.p.s. (FIG.1).

A sponge is dipped into the slurry so that the slurry is adsorbed intothe sponge. After taking out the sponge, the slurry in the gaps of thesponge is squeezed out using a roller. Then, the sponge is dried in anoven for 20 hours at 55° C.

The sponge is fired in a biscuit furnace by raising the temperature from100° C. to 350° C. at a rate of 2° C./min. The temperature is maintainedat 350° C. for 2 hours, then lowered at a rate of 10° C./min. Thecomposite ceramic is sintered in a nitriding furnace, where thetemperature is raised to 1250° C. at a rate of 30° C./min. After 5hours, the temperature is raised to 1350° C. and kept at thattemperature for 10 hours.

The XRD spectrogram of the porous ceramic filter thus produced after the15 hours sintering process is shown in FIG. 2, wherein α(low-temperature form) denotes the alpha-form and β (high-temperatureform) denotes the beta-form of Si₃ N₄. It can be seen from FIG. 2 thatno silicon remains in the Si₃ N₄ ceramic filter since the nitrition ofsilicon powder into Si₃ N₄ is complete. Furthermore, the purity the Si₃N₄ ceramic filter is almost 99%, and hardly any shrinkage of the filtercan be found after the sintering process.

While the preferred embodiment has been described, variations theretowill occur to those skilled in the art within the scope of the presentinventive concepts which are delineated by the following claims.

What is claimed is:
 1. A process for manufacturing a porous Si₃ N₄ceramic filter comprising:mixing silicon powder, water and methylcellulose to obtain a slurry, dipping a sponge into the slurry so thatthe slurry is adsorbed to the sponge, drying the sponge, biscuit firingthe sponge, and sintering the sponge in a nitriding furnace for about 15hours at temperature in a range of 1200°-1400° C. to form the porous Si₃N₄ ceramic filter.
 2. A process as claimed in claim 1, wherein theslurry is 65-70 weight percent in dry solid content the silicon powder,and 0.25 to 0.3 weight percent of the methyl cellulose based on a totalweight of the water.
 3. A process as claimed in claim 1, wherein thesponge is made of thermal plastic material.
 4. A process as claimed inclaim 1, wherein the sponge is dried in an oven at 55° C. for 20 hours.5. A process as claimed in claim 1, wherein the sponge is biscuit firedat a temperature of 350° C. for 2 hours.
 6. A process as claimed inclaim 1, wherein the sponge is sintered in the nitriding furnace at1250° C. for 5 hours, and at 1350° C. for 10 hours.